A. Field of the Invention
The present invention relates to the method for making a small hole in a plate member that is employed in fluid flow regulating applications, such as in an automatic transmission, and the invention further relates to the plate member having such a hole formed therein.
B. Description of the Related Art
A control valve body which is used in an automatic transmission of an automobile has a structure similar to that shown in FIG. 3. In FIG. 3, an upper body 2 and lower body 3 of the valve body 1 are connected to one another with a separate plate 5 interposed therebetween. Channels are formed in the bodies 2 and 3 defining oil passages 6 and 7, respectively. The separate plate 5 is attached to the upper body 2 and the lower body 3 tightly in a liquid-sealed condition. The separate plate 5 has a plurality of oil passing holes 9 formed therein, but only one oil passing hole 9 is shown in FIG. 3 connecting the oil passages 6 and 7. The diameter of the oil passing hole 9 and the flatness of the plate surface at the edge of the oil passing hole 9 are important for proper fluid regulating characteristics of the oil passing hole 9 of the separate plate 5.
The oil passing hole 9 has a predetermined diameter and is typically formed by press working the separate plate 5 using a punch, such as a punch 10 shown in FIG. 4.
There are problems associated with the hole forming processes in the above described separate plate 5. When the thickness T of the plate 5 is small, for example, 2 mm, and the diameter D of the oil passing hole is less than 50% of the thickness T (less than 1 mm), a swollen part 11 is generated during press working. The swollen part 11 is generated by deformation of metal during the hole making procedure using the punch 10. The swollen part 11 is usually formed on the side of the plate 5 where the punch 10 first engages the plate 5 in the hole forming process. Since the hole 9 is round and the swollen part 11 is formed all around the hole 9, the swollen part has an annular shape on the surface 12. Occasionally a similar swollen part is formed on the under surface 13 resulting in, for instance, a flash 14 being formed on an under surface, the under surface being a surface that the punch 10 moves through after completely extending through the plate 5 during the hole forming process.
If the control valve body is assembled and includes the plate 5 where the plate 5 includes the swollen part 11, the liquid-tightness of the oil passages 6 and 7 is reduced because the upper body 2 and the lower body 3 may engage the swollen part 11 thus making fluid leaks possible.
In addition, the process as shown in FIG. 4 has a drawback. Specifically, a ratio is defined between the diameter D and the thickness T of the plate 5. If the ratio is very small, where the thickness T is much greater than the diameter D, then punch 10 might not be able to withstand the forces necessary to punch through the plate 5. In other words, the punch 10 might break if the ratio is very small. Therefore, with a large thickness T and a relatively small diameter D, the punch 10 does not have a long usable life.
Another example of a portion of control valve body for an automobile automatic transmission as shown in FIG. 14. As with the previous example above, in FIG. 14, an upper body 2 and lower body 3 of a valve body 1 are connected to one another with a separate plate 105 interposed therebtween. Again, channels are formed in the bodies 2 and 3 which define oil passages 6 and 7. The separate plate 105 is connected with the upper body 2 and lower body 3 in a liquid-tight condition and a plurality of oil passing holes 109 are formed therein. Only one hole 109 is shown in the plate 105 connecting the oil passages 6 and 7 in FIG. 14.
Depending on the specific functions of control valve body, the oil passing hole 109 may include a taper face 111, as shown in FIG. 14. In detail, the inner circumferential face of the oil passing hole 109 includes the taper face 111 which has an outer edge 110 and an inner edge 112. The outer edge 110 defines a diameter that is much larger than the diameter of the inner edge 112 and the hole 109 itself. Further, the hole 109 defines an inner cylinder surface 113 that extends from the inner edge 112 to the under side of the plate 105. The taper face 111 is an inclined surface with a generally conical shape overall. The taper face 111 is often necessary when putting a ball or check valve in place above the taper face 111.
Since the oil passing hole 109 with the taper described above is required to have a precise predetermined size, it is often formed by utilizing a counter sink drill. Such a drilling method has a drawback in that productivity is low because the drilling process takes a significant amount of time, in particular when large quantities of the plate 105 must be produced.
Further, when the taper surface 111 is made by press working, productivity may be increased but a variety of additional drawbacks accompany the increase in productivity.
As shown in FIG. 15, the oil passing hole 109 is made by press working. First, a raw plate material (which later forms the separate plate 105) is held between upper and lower die plates 115 and 116. A guiding hole is formed in the plate material (the hole later becomes the hole 109). The taper surface 111 is made by plastic working the raw material using a punch 117 which is forced into the guiding hole of the die plate 115.
The punch 117 has a taper part 118 with a taper surface corresponding to the taper face 111, and a column-like protrusion 119 to form a cylinder surface 113 projects from the end of the taper part 118. The taper part 118 extends in a taper to the outer circumferential surface of the punch 117, and an outer diameter of a large diameter base part 120 of the punch 117 is much larger than an outer diameter of a large diameter edge 110 of the taper surface 111 of the oil passing hole 109. Therefore, an annular gap 122 with a triangle cross section is defined around the oil passing hole 109 between the base part 120 of the taper part 118 and the upper surface 121 of the separate plate 105. As the result, the press working deformation causes a movement of the material and a swelling part 125 is formed on the upper surface 121 around the oil passing hole 109.
A movement of the material is also caused on the lower surface 127 of the separate plate 105 and a swelling part 126 is formed around the oil passing hole 9. The swelling part 126 is generated as a result of elastic deformation caused by the upper end surface of the die plate 116. It is also found, as shown in FIG. 15, that the swelling parts 25 and 26 tend to be generated when an open angle .THETA. of the punch 117 is greater than 80.degree. and the ratio (M/T) of a depth M of the taper surface 111 and a thickness T of the separate plate 105 is more than 0.5.
Such swelling parts 125 and 126 cause a deviation of flow characteristics of oil in a valve in FIG. 14. Consequently, it sometimes happens that the desired characteristics of sitting on the bearing surface can not be obtained when the taper surface 111 is used as a bearing surface for a ball valve.
In addition, when a gasket 128 is interposed both between the upper body 2 and the separate plate 105, and another gasket 128 is interposed between the lower body 3 and the separate plate 105 as shown in FIG. 16, the swelling parts 125 and 126 make an indentation in the gasket 128. Therefore, there is a possibility that sealing capabilities of the gasket 28 may be compromised.
It should be noted that in the above discussed drawings, the swelling parts have been exaggerated somewhat in order to better demonstrate the shortcomings of the prior art and in actual examples may not have the depicted size when compared to the overall thickness of the plate member.